Shielded cable

ABSTRACT

A SHIELDED CABLE IN WHICH ONE OR MORE INSULATED CONDUCTORS ARE COVERED BY A PLURALITY OF LAYERS OF SPIRALLED METALLIC TAPE. EACH TAPE HAS A DIFFERENT LAY, AND ONE OR MORE TAPES ARE SPIRALLED IN ECH DIRECTION. AS A RESULT ANY STRAY CURRENTS INDUCED IN THE TAPES HAVE CANCELLING FIELDS, AND SO SUBSTANTIALLY NO CURRENT IS INDUCED IN THE INNER CONDUCTORS.

United States Patent [72] Inventor Loren 11. Hutchins, Jr.

Stoneham, Mass. [21] Appl, No. 774,466 [22] Filed Nov. 8, 1968 [45] Patented June 28, 1971 [73] Assignee Simplex Wire and Cable Company Cam bridge, Mass.

[54] SHIELDED CABLE l0 Chlms, 2 Drawing Figs.

[51] lnhCl 1101b 7/32 [50] Field of Search 174/106,

[56] References Cited UNITED STATES PATENTS 2,152,706 4/1939 Mougey l74/36X Primary Examiner- Lewis 1-1. Myers Assistant ExaminerA. J. Grimley Attorney- McLean, Morton and Boustead ABSTRACT: A shielded cable in which one or more insulated conductors are covered by a plurality of layers of spiralled metallic tape. Each tape has a different lay, and one or more tapes are spiralled in each direction. As a result any stray currents induced in the tapes have cancelling fields, and so sub stantially no current is induced in the inner conductors.

PATEN'TED JUN28 197i INVENTOR LOREN H. HUTCHINS, JR.

SHIELDED CABLE The present invention pertains to electrical cable. More particularly the present invention pertains to shielded twinaxial electrical cable having low susceptibility to crosstalk and to induced currents from such phenomenon as lightning.

In numerous applications a pulse type electrical signal must be transmitted an appreciable distance by cable. In many such applications it is essential that no spurious pulses be induced in the conductor which might otherwise be mistaken for signal pulses. For example, intelemetering systems used to transmit information or data, pulse coding is frequently used. Should a stray pulse be induced in the cable, it would interfere with the message transmission, anderroneous data would be received. During the transmission of security classified messages it is necessary to prevent interception or bugging of the message by unauthorized persons. In addition, such cable is frequently utilized to apply a detonating pulse to an explosive charge. Should a stray pulse be induced in the cable, the charge would be exploded erroneously.

Ordinary shielded cable comprising one conductor within and insulated from a second encircling conductor has not been found satisfactory for many applications. Although such shielded cable does considerably reduce spurious signals and is satisfactory for many applications such as transmission of commercial television signals, the large amount of crosstalk or noise in such cables makes it unsatisfactory for many purposes. In addition the susceptibility of such cable to bugging makes it unsatisfactory for transmission of security classified messages. While a solid metallic tube might provide perfect shielding, such a tube is inflexible and thus is impractical.

The present invention is a cable for use in applications in which it is essential that substantially no erroneous pulses be induced and that the message not be illicitly intercepted. The cable of the present invention includes a highly balanced pair of conductors within an insulating sheath and surrounded by a multilayer shield. Each layer of the shield is formed of a thin metallic tape or foil and is wrapped about the conductor pair with a spiral lap. The amount ofoverlapping and the lay of the metallic tape varies between the different layers in such a manner that currents induced in the shield produce cancelling electromagnetic fields. As a consequence, substantially no current is induced in the conductor pair, and yet the finished cable is sufficiently flexible to permit storage in rolls.

These and other aspects and advantages of the present invention are apparent in the following detailed description and claims, particularly when read in conjunction with the accompanying drawings: In the drawings:

FIG. I is a partially broken fragmentary view of a cable in accordance with the present invention; and

FIG. 2 is a view taken along the lines 2-2 of FIG. 1.

As depicted in FIGS. 1 and 2 cable includes two balanced insulated conductors l2 and 14. In FIG. I conductors l2 and 14 are each depicted as comprising several strands of conductive material such as copper wire. While this multistrand conductor is particularly preferred where the cable is subject to flexing, this invention is likewise suited for use with cable having single strand conductors. Each of the conductors 12 and 14 includes an insulating outer cover ofa material such as polypropylene. Conductors l2 and 14 are carefully balanced and are twisted about a common axis so that the two conductors are nearly identical and so that any currents induced in the two conductors cancel. During the twisting operation care must be taken to assure that the balance is not disturbed. Conductors 12 and 14 are then surrounded by an insulating jacket 16, which by way of example might be of polyethylene. Jacket 16 has a circular cross section and is carefully centered on the axis about which conductors I2 and 14 are spiralled.

The outer surface of jacket I6 is covered by a soft metallic jacket I8 made up of a thin metallic tape spirally wrapped about jacket 16 in an overlapping manner with a lay in a first direction. The outer surface of jacket 18 is covered by a metallic jacket 20 made up of several narrow metallic tapes butted together and spirally wrapped with a lay in a second direction opposite that of the lay of jacket 18. Jacket 20, in turn, is covered by a spirally wrapped soft metallic jacket 22 made of a thin metallic tape having width in the order of the width of the tape forming jacket 18. The consecutive turns of the spiral wrap overlap and are layed in the second direction,

as is jacket 20. The outer surface ofjacket 22 is covered by a fourth metallic jacket 24 made up of several spirally wrapped thin metallic tapes butted together and wrapped about jacket 22 with a lay in the first direction. Jacket 24 is then covered by an outer metallic jacket 26 made up of a thin metallic tape, having a width in the order of the width of tapes l8 and 22, and spirally wrapped with overlapping turns with a lay in the second direction. Outer insulating cover 28 then covers metallic jacket 26 and is formed of a suitable insulating material such as polyurethane.

The several tapes forming jackets 18, 20, 22, 24, and .26 might be of any electrically conductive materials, such as copper, aluminum, steel and clad metals, for example. Due to the opposite spiral of the several jackets 18-26, any stray currents induced in those jackets produce cancelling fields, and so no net current is induced in conductors l2 and 14 by such stray currents; Since some metals lack a shielding effect at particular frequencies, instead proving to be a window to such frequencies, it may be desirable to utilize different metals in the different tape jackets. Alternatively, if it is known that only one frequency is to be expected, then all of the tape jackets might be of same material which does not provide a window to that frequency. The consecutive turns of each spiral wrap are touching in that they either overlap or are butted together.

In one cable constructed in accordance with the present invention the following materials were utilized. Conductors I2 and 14 were each formed ofseven strands of No. 26 AWG soft copper wire with a left-hand lay and were each insulated by a 0.09 inch diameter coating of polypropylene. The two insulated conductors were then twisted with a 2.] inch left-hand lay and were belted and filled with a high molecular weight polyethylene and covered by jacket 16 of like material and having a diameter of 0.30 inch and a wall thickness of 0.04 inch. Jacket 18 was formed of one 0.003 inch by 0.75 inch soft copper tape wrapped with approximately a 22 percent lap at a 0.75 inch right-hand lay. Jacket 20 was formed of six 0.015 inch by 0.158 inch soft copper tapes butt wrapped with a 3.16 inches left-hand lay. Jacket 22 was formed of one 0.003 inch by 0.75 inch soft copper tape wrapped with approximately an 18 percent lap at a 0.75 inch left-hand lay. Jacket 24 was formed of six 0.0l5 inch by 0.176 inch soft copper tapes butt wrapped with a 3.52 inches left-hand lay. Jacket 26 was then formed of one 0.003 inch by 0.75 inch soft copper tape wrapped with approximately 15 percent lap and a 0.75 inch left-hand lay. Insulating jacket 28 was formed of a black polyurethane with a minimum wall thickness of 0.05 inch, giving a finished cable with an outer diameter of 0.525 inch. This finished cable has a flexible copper shield with a greater area of copper than can be practically obtained with multiple braids'of insulation.

lclaim:

l. A shielded cable comprising insulated electrical conductor means; a first metallic tape means spirally wrapped about said insulated electrical conductor means with a first direction of lay; a second metallic tape means spirally wrapped about said first metallic tape means with a second direction of lay; a third metallic tape means spirally wrapped about said second metallic tape means with said first direction of lay; and a fourth metallic tape means spirally wrapped about said third metallic tape means with said second direction of lay; one of said first and third and one of said second and fourth metallic tape means each individually comprising a plurality of metallic tapes butted together and the other two said metallic tape means each individually comprising a single metallic tape.

2. A shielded cable as claimed in claim I in which the consecutive turns of each of said other two noncontiguous ones of said metallictape means overlap.

3. A shielded cable as claimed in claim 2 in which. the amount of overlap is from about percent to about 22 percent of each tape width.

4. A shielded cable as claimed in claim 1 in which each of said plurality of metallic tapes has a width in the range of about 0.158 inch to about 0.175 inch and in which said other two noncontiguous ones of said metallic tape means each individually comprises a single metallic tape having a width of about 0.75 inch.

5. A shielded cable as claimed in claim 1 in which said first two noncontiguous ones of said metallic tape means each are spirally wrapped with a lay of from about 3.16 inches to about 3.52 inches and said other two noncontiguous ones of said metallic tape means each are spirally wrapped with a lay of about 0.75 inch.

6. A shielded cable as claimed in claim 1 in which said insulated electrical conductor means comprises a balanced pair of insulated electrical wires.

7. A shielded cable as claimed in claim 6 in which each wire ofsaid balanced pair is a multiconductor wire.

8. A shielded cable as claimed in claim 6 further comprising a fifth metallic tape means spirally wrapped about said fourth metallic tape means.

9. A shielded cable as claimed in claim 1 further comprising electrical insulating means covering said fourth metallic tape means.

10. A shielded cable as claimed in claim 1 in which each metallic tape has a thickness of from about 0.003 inch to about 0.015 inch. 

